Allow typeahead selection of Firewall macros

This fixes the bug: https://bugzilla.proxmox.com/show_bug.cgi?id=677

Remarks:
* allowBlank is already set in the class definition
* The value of the comboBox can be either '' after the component initial
load, or 'null' if the user selected a macro and cleans the field afterwards.
This is the reason of the if (!value) test which will catch empty strings and
null.

www/manager/grid/FirewallRules.js

@@ -164,10 +164,10 @@ Ext.define('PVE.FirewallRulePanel', {
 		name: 'macro',
 		value: '',
 		fieldLabel: gettext('Macro'),
-		allowBlank: true,
+		editable: true,
 		listeners: {
 		    change: function(f, value) {
-                        if (value === '') {
+                        if (!value) {
 			    me.down('field[name=proto]').setDisabled(false);
 			    me.down('field[name=sport]').setDisabled(false);
 			    me.down('field[name=dport]').setDisabled(false);